If Elon Musk gets his way, humans will populate Mars by 2020. Just don’t drink the water.

Much worse than traveler’s diarrhea, you may be infected by a millenia-old sentient virus.

When the Flood—imprisoned in a glacier on Mars thousands of years ago by Ice Warriors—is unwittingly released by the crew of Bowie Base One, the Doctor must decide whether to use his knowledge of their fate to change history.

The Martian species, whose natural state is liquid water, needs a physical form to survive. In this case, the seven-strong team of human colonists; homegrown heroes sent to the red planet for a five-year exploration mission.

But thanks to an improperly fitted water filter, the pathogen easily spreads through the first human community on the planet (named after the late David Bowie, who famously asked is there life on Mars?), taking lives one by one—starting with officers Maggie Cain (Sharon Duncan Brewster) and Andy Stone (Alan Ruscoe), the latter contaminating Dr. Tarak Ital (Chook Sibtain).

The infection is not hard to spot: cracked skin around the mouth, ghoulish growling sounds, water pouring from various orifices. Just one drop, and you’ll almost instantly develop blackened teeth, faded blue eyes, internal fission, and a pretty nasty case of hypersalivation.

A patient and intelligent pathogen, the Flood, like any galactic threat, does have its weaknesses: Electricity, for one (“Water and electricity: a bad mix”). Heat, nuclear explosions, and freezing temperatures, as the Ice Warriors proved, also help contain the microbe.

Despite the crew’s best efforts to suppress the surge—now leaking through allegedly air-tight cabins and “10 feet of steel combination”—deputy Ed Gold (Peter O’Brien) and technicians Steffi Ehrlich (Cosima Shaw) and Roman Groom (Michael Goldsmith) become saturated, joining the Flood hivemind in what looks like some sort of ritual water sacrifice.

The Tenth Doctor must make an important decision (via BBC)

The Doctor, usually a real stickler for not screwing with fixed points in time, watches and listens as the increasingly moist scene plays out.

According to historical records, the base is destroyed by a nuclear blast, inspiring future generations to explore the universe and meet interstellar life forms.

But the Time Lord leaves no man, woman, or robot behind. And after coming to the imperious realization that “the laws of time are mine, and they will obey me,” he returns to save the remaining team members: nurse Yuri Kerenski (Aleksandar Mikic), geologist Mia Bennett (Gemma Chan), and Captain Adelaide Brooke (Lindsay Duncan, of Mansfield Park, Alice in Wonderland, Birdman, Sherlock).

The Doctor and Captain Brooke uncover the mystery of the Flood (via BBC)

The drooling zombies are enough to give you nightmares for at least a week. But if you thought these dripping victims are terrifying, just imagine the original concept drawings, which, according to BBC America, were deemed “too scary” for the intended audience.

The finished product, which boasted the same special effects as a ’90s slasher flick, was actually a pared-down version of writer Russell T Davies initial concept. A planned effect of liquid dripping down the creatures’ faces made it impossible for actors to deliver their lines, as water kept getting in their eyes. Costume designers were forced to reconfigure the masks, so water emerged only from the mouth.

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